1. Field of the Invention
The present invention relates to molding compositions, to methods of making such compositions, to products made from said compositions, and to methods of making said products. In another aspect, the present invention relates to irradiated molding compositions, to methods of making such compositions, to products made from said compositions, and to methods of making said products. In even another aspect, the present invention relates to molding compositions having improved environmental stress cracking resistance and environmental fatigue resistance, to methods of making such compositions, to products made from said compositions, and to methods of making said products
2. Description of the Related Art
Stress cracking or environmental stress cracking (ESC) is the brittle failure of plastic parts when simultaneously subjected to static mechanical stress and chemical exposure. In a similar fashion, environmental fatigue (EF) is the failure or cracking of a part when simultaneously subjected to dynamic mechanical stress and chemical exposure. Insufficient environmental stress cracking resistance or environmental fatigue resistance leads to greatly shortened service life of a part.
Flexible molded articles are used in various packaging, dispensing, pumping, footwear, or protective mechanical boot applications. Ideally, these flexible molded articles will retain essentially the same mechanical properties during the desired service life. However, in a less than ideal world, especially mechanical service in the presence of aggressive chemical environments, such exposure can attack or alter these flexible molded articles and hence the performance or service life of such articles.
It is desired that these flexible molded articles withstand, without cracking, mechanical cycling during exposure to a wide range of chemicals. These articles preferably retain dimensional stability after long periods in a compressed state. Dimensional stability permits the springing back of said articles toward their natural uncompressed position as opposed to becoming set to the compressed shape. Additionally, when these articles are held under compression in contact with chemicals and for an extended period of time, it is desired that they withstand environmental stress cracking that could create cracks or openings through the wall of the part and/or alter the mechanical properties.
It is desirable that the flexible molded articles be suitable for service in a wide range of chemical environments. However, designing a suitable polymeric material for such a flexible molded article useful in a wide range of chemical environments has proven difficult.
For example, numerous elastomeric materials can be used to make flexible spring-like moldings that satisfy mechanical design criteria in the absence of an aggressive chemical environment, or in one particular type of aggressive chemical environment. For example, a material that is suitable for an acidic solution can be ill-suited for an alkaline or oxidizing medium; a material that is resistant to stress cracking in a dilute alcoholic solution can crack after the addition of a perfume, for example, a terpene-based perfume. More specifically, thermoplastic elastomers such as polyesters and polyamides provide spring-like behavior in an article. However, these materials lack chemical resistance to extreme pH conditions because of chemical degradation. Thermoplastic urethane elastomers provide a wide range of mechanical properties and could satisfy the mechanical design requirements of a flexible spring-like device. However, these materials can degrade from exposure to alkaline solutions. Resins such as poly(vinyl chloride) or propylene and styrene-ethylene-butylene-styrene block copolymer blends, when highly plasticized, exhibit greater chemical resistance but lack either the compression set resistance or dynamic response required for a flexible spring-like device. Low crystallinity or low density polyethylene or ethylene copolymers typically provide favorable moduli and good chemical resistance. However, these materials can often undergo environmental stress cracking unless very high molecular weight resins, that are ill-suited for injection molding, are used. Thus, selection of a material for a flexible spring-like article for use in a wide variety of chemical environments, preferably aqueous solutions or emulsions, becomes difficult.
Alternatively, if several materials are chosen for particular types of chemical service, then fabrication costs can be large. For example, because each material can exhibit unique shrinkage during molding, achieving dimensional tolerances often requires the fabrication of individual molds that are tailored to the shrinkage of each material. Accordingly, use of several molds to accommodate the shrinkage of a variety of thermoplastic materials for the production of the same part but for service in different fluids leads to higher fabrication costs. It is desired to minimize this cost by use of a few materials as possible, ideally, by the use of one material.
One type of application of particular interest, is the use of a flexible spring-like device, such as a bellows, in chemical service. Techniques for fabricating a bellows are known in the art. For example, U.S. Pat. No. 5,236,656, issued Aug. 17, 1993 to Nakajima discloses a method of injection blow molding synthetic resin bellows. U.S. Pat. No. 5,439,178, issued Aug. 8, 1995 to Peterson, discloses a pump having a bellows which can be constructed from polyolefins such as polypropylene, low density polyethylene, ethylene vinyl acetate, rubber and thermoplastic elastomers. World Patent No. WO 88/06088 to Cheynol et al., published Aug. 25, 1988, discloses a process and apparatus for the manufacture of a protective bellows for a transmission device, in which rough molded bellows are injected during a primary stage into a first mold, then, in a second mold, the rough molding is blown rib by rib, or by groups of ribs, until the required form of the bellows is achieved.
However, these patents do not address the fabrication of a bellows suitable for use in a wide variety of chemical environments.
Specifically, for a flexible spring-like device in chemical service, an enhancement of one or more material properties is required for any material to satisfy the requirements of such service.
Methods to enhance the environmental stress cracking resistance of, for example, polyethylene and ethylene copolymers are known. These methods include crosslinking of polyethylene by peroxides or irradiation. Crosslinking with peroxides has disadvantages such as increased viscosity, localized scorching, and health concerns. Crosslinking by irradiation avoids these disadvantages.
Irradiation of thermoplastics is known in the art as shown by the following prior art references.
U.S. Pat. No. 2,855,517, issued Oct. 7, 1958, to Rainer et al., discloses irradiation of polyethylene above its melting point into a clear, transparent liquid, followed by quenching to preserve its transparency and thus produce transparent polyethylene.
Lanza, V. L., in "Effect of radiation on polyethylene", Modern Plastics, vol. 34, No. 11, pp. 129-132, 134 and 136 (1957), discloses use of high energy electrons, gamma rays, or atomic pile radiation to cross-link polyethylene, resulting in greater insolubility, changes in tensile properties, increase in the environmental stress cracking resistance, and better stability at higher temperatures.
U.S. Pat. No. 2,906,678, issued Sep. 29, 1959 to Lawton et al., discloses a process for irradiating polyethylene at elevated temperatures to enhance crosslinking efficiency.
Olander, John, in "A guide to radiation equipment", Modern Plastics, vol. 38, No. 10, pp. 105-106, 109, 110, 113, 116, 119, 190 and 192 (1961), discloses use of high velocity electrons, X-rays, and gamma rays in the cross-linking of polymeric materials, which in the case of polyethylene, can result in increased temperature and solvent resistance, increased tensile strength, increased melt viscosity, decreased elongation, higher resistance to deformation under load, and improved environmental stress cracking.
U.S. Pat. No. 3,102,303, issued Sep. 3, 1963 to Lainson, discloses an apparatus for irradiation of plastics, in which extruded polyethylene pipe is subjected to controlled ultraviolet radiation during extrusion to cause crosslinking.
U.S. Pat. No. 3,130,139, issued Apr. 21, 1964 to Harper et al., discloses polyolefin articles containing uniform dispersions of carbon black in polyolefins made by subjecting the polymer to ionizing radiation prior to its admixture with the carbon black, which articles can be irradiated to further improve chemical and physical properties.
U.S. Pat. No. 3,563,870, issued Feb. 16, 1971 to Tung et al., discloses a process for producing injection molded parts, in which prior to molding the olefin polymer particles are exposed to high energy radiation ranging from about 0.05 to about 0.3 megarads. Tung further teaches that above 0.3 megarads the melt extensibility is decreased.
U.S. Pat. No. 3,734,843, issued May 22, 1973 to Tubbs, discloses the initial melt index of ethylene/vinyl acetate copolymers can be substantially lowered by treating the copolymers with high energy ionizing radiation of high dose rates up to a critical dose, i.e., point at which gel is formed. Tubbs further discloses that for polymers having an initial melt index of less than 50, a dose of 5 megarads will cause significant gel, and for polymers having an initial melt index of 10 or less, a critical dose will rarely exceed 2 megarads.
U.S. Pat. No. 3,773,870, issued Nov. 20, 1973 to Spillers, discloses that a substantially uniform dosage of ionization can by applied to tubing if the tubing is completely flattened in the zone of radiation. This is accomplished by passing the tubing over a flat roller directly under the electron beam generator, or alternatively, by passing the tubing through a pair of vertically spaced flat plats arranged directly under the electron beam generator.
U.S. Pat. No. 3,783,115, issued Jan. 1, 1974 to Zeppenfeld, discloses a process for preparing cross-linked polyethylene fiber by irradiating a polyethylene sheet material with a high does rate and thereafter heat tempering the irradiated sheet material.
U.S. Pat. No. 4,049,757, issued Sep. 20, 1977 to Kammel et al., discloses the production of shaped bodies by extrusion, injection molding or extrusion pressing of a crosslinkable polymer, which shaped bodies are then subjected to high-energy radiation to crosslink the polymer.
U.S. Pat. No. 4,264,661, issued Apr. 28, 1981 to Brandolf, discloses a process for manufacturing an irradiation crosslinked molded article. The process includes molding a thermoplastic to form an article which does not crosslink under the molding conditions, removing the article from the mold with a separable portion of the mold being retained by the molded article, irradiating the article to crosslink it, and finally, removing the separable portion from the article.
U.S. Pat. No. 4,367,186, issued Jan. 4, 1983 to Adelmann et al., discloses a process of preparing molded articles from crosslinkable aromatic thermoplastic polycarbonate by first subjecting the polycarbonate to irradiation with UV rays or other high energy rays thereby causing crosslinking of about 5 to about 50 weight percent. The polycarbonate is extruded in a known manner and irradiated, either immediately as the extruded strand or in the form of granules, and are then made into any desired shape by extrusion or injection molding.
U.S. Pat. No. 4,582,656, issued Apr. 15, 1986 to Hoffman, discloses the use of additives for polyolefins to prevent the occurrence of undesirable accumulations and discharges of electrical charge concentrations and/or to suppress or eliminate the undesirable foaming of polyolefin molded articles during irradiation.
U.S. Pat. No. 5,061,415, issued Oct. 29, 1991 to Depcik, discloses a process for improving the quality of injection molded parts by utilizing a high frequency electromagnetic field and application of pressure in sunk spots in critical regions, which for partially crystalline material, at an operating temperature of more than 10.degree. C. above the melting point, and which for amorphous material, at an operating temperature more than 30.degree. C. above the glass transition temperature.
JP 098850, published Dec. 27, 1991, discloses production of imide silicone-type polymer articles by molding imide silicone-type polymer into the desired shape and irradiating it with an electron beam in the presence of crosslinking aids, to produce sheets, pipes and insulated wires having improved heat distortion resistance and solvent resistance.
However, in spite of these advancements in the above prior art of numerous irradiation methods for enhancing some properties of thermoplastics, none of the above prior art references disclose or suggest how to enhance the environmental fatigue life for a flexible product in aggressive chemical service.
Thus, there is still a need for a material useful in aggressive chemical service.
There is another need in the art for a method of making such a material that is useful in aggressive chemical service.
There is even another need in the art for flexible articles useful in aggressive chemical service.
There is still another need in the art for a method of making flexible articles useful in aggressive chemical service.
These and other needs in the art will become apparent to those of skill in the art upon review of this specification, including its drawings and claims.